Hot kilns are used in carrying out a large number of economically important processes.
Owing to the nature of the process for which they are used such kilns may attain lengths as great as six hundred feet and be supported by annular tires carried on rollers, mounted upon piers as high as seventy feet above the ground.
The steel vessel constituting the kiln is relatively thin walled, being usually lined with a refractory lining to protect the walls of the vessel and to provide a protective thermal gradient to the kiln. The kiln shell is quite flexible, as a consequence.
Owing to the size of such kilns the daily throughput is of such value that shutdown of a kiln is to be avoided at all costs.
The construction of high temperature kilns necessitates provision being made for expansion of the shell, relative to its supporting tires. For this reason the tires generally fit loosely on the shell. The "looseness" of the arrangement is further complicated by wear that takes place in the supporting rollers, on which the tires are carried, and the susceptibility of the supporting piers, in many instances, to swaying during operation of the kiln.
As a consequence of these and other factors such kilns get out line, in that intermediate portions of the kiln do not rotate coaxially with other portions of the shell. This misaligned condition introduces unnecessary, but frequently unavoidable stresses, particularly in the thin walled shell, which are potentially destructive thereto.
In order to ameliorate this condition it is the aim of many existing methods to determine the center of rotation at differing axial locations along a kiln, to permit compensating adjustment to be made to the rolls on which the kiln tires are supported, without shutting the kiln down, so as to bring the kiln into more close approximation of a single rotational axis.
The foregoing enunciated difficulties are compounded by the fact that kiln shells frequently exhibit dynamic ovality, in the running of the flexible shell within the stiffer tire.
Prior methods include sighting off side vertical tangents and the bottom dead center of the tire, but could not effectively compensate for uneven wear over both the tires and the supporting rollers. Wear also takes place between the tire and its supporting pads, or the tire and the shell, which wear may destroy the concentricity of the construction.
The importance of an effective on-stream alignment measuring scheme is that, if of sufficient accuracy, it permits effective preventive maintenance to be carried out, to minimize kiln wear and damage.
Certain prior art hot kiln alignment measurement schemes exist, such as "Alignment of Rotary Kilns and correction of Roller Settings During Operation", B. Krystowczyk, Bromberg, Poland 1983, published Zement-Kalk-Gips Translation ZKG No. 5/83 (p.p. 288-292). This method uses an optical plumb to sight off vertical tangents to the kiln tires. The method suffers from inaccuracies due to variations in the tire to shell clearances.
The method is totally manual, and requires working closely adjacent to hot kiln surfaces, and is limited by human response times in the rate of taking readings as the kiln rotates.
In the case of faster rotating hot calciner kilns these can prove to be serious drawbacks. The method also requires the simultaneous taking of readings by three individuals, which again limits both speed and accuracy of applying the method.
The method further required a determination of the gaps existing between the tires and the kiln shell at the respective measuring spots, if desireable accuracy is to be achieved, as it is an improvement to the trueness of the shell to which the process is usually directed.
Another process involves the use of a laser theodolite and a second theodolite having their outputs connected with a computer. The laser theodolite is focussed at a point on the face of the surveyed tire, and the second theodolite, from a different location, is also focussed on the laser illuminated spot. The computer digests the respective angles of the theodolites and provides three dimensional x.y and z axis coordinates as the address for the instantaneous target, during rotation of the kiln. In addition to requiring multiple vantage points for viewing the tire, this method requires that the instruments be set up and calibrated a number of times, relative to a selected, single originating point. This system appears related to a similar system that has been used with considerable advantage in erecting large static structures such as chimney stacks, buildings and rocket launchers.
However, its adaption to a dynamic target such as a kiln wherein the supporting piers may be moving as a consequence of the dynamic and shell reaction forces generated, has been less than straightforward. The time required to set up the system is somewhat prohibitive, and the results achieved are barely adequate. Thus, the cost and complexity of this prior system has limited its applicability and popularity, with regard to kiln hot alignment.
A yet further process apparently adopted in response to the Krystowczyk method includes the use of plumb lines draped over the rotating tires, to determine their positions as vertical tangents relative to an established centre line datum.
The adoption of such manipulations has tended to reduce the credibility of hot alignment of kilns in the eyes of users.
In considering the prior art systems, it will be understood that kiln internal temperatures as high as 3000 degrees F. require that measurements to be made external to the kiln.
Most prior methods basically rely upon external procedures, for measurements involving measuring the diameter of the kiln supporting tires; the diameter of the tire supporting rolls; the gaps between the tire and kiln shell; and, the spacing between the respective supporting rolls. Using these measured values the location of the kiln center is establishes geometrically.
However, it must be born in mind that typically the kiln tires may be as wide as two to three feet axial width, and the supporting rollers may be three to four feet in axial width. However, these items wear in service, the tires becoming convex surfaced, the rollers concave surfaced. As a consequence, the accuracy and constancy of measurements is highly suspect. Also, the kiln structure is temperature sensitive, so that thermal changes may effect significant variations in the relationships between the respective moving parts, some of which are directly influenced by kiln temperature, and other, such as the supporting rollers, much less so.
In further considering the background to kiln operation, including implications stemming from their design, it will be appreciated that the kiln supports, located at selected positions along its length, are intended to achieve even loading. Factors such as variations in refractory lining thickness, due to different temperatures and wear rates, variations in shell plate and tire thicknesses, non-uniformity in the travelling kiln load, variation in the thickness of internal coating of the refractory etc., may cause variations in load shell stiffness and ovality, and changing deflections at the supports which generally develop during the operation of a kiln.